Effects of prolactin and growth hormone on growth and metamorphosis of tadpoles of the frog, Rana pipiens.

Mammalian prolactin at doses of l-50 pg/day promoted growth (increased wet weight, dry weight and body length) in Rana pipiens tadpoles. Mammalian growth hormone (GH) promoted growth only at the higher doses (50 pg/day and above) and was not so effective as prolactin. Prolactin also inhibited metamorphosis (at doses of 5 pg/day and above). Inhibition of metamorphosis did not appear to be due to reduction in tissue sensitivity to thyroxine since prolactin-treated tadpoles metamorphosed in response to exogenously administered thyroxine (4-100 #g/liter) to the same extent as controls. Prolactin promoted further growth in tadpoles in which the thyroid level had been reduced with thiouracil or by thyroidectomy. This indicates that growth promotion caused by prolactin is not just the result of thyroid inhibition, although it is possible that a portion of the growth stimulation is due to t?i8 sntithvroid action of prolactin. .,. ‘. ,, < _::.